The present invention relates to electrical circuitry and more particularly a class of circuitry known as a maximum voltage source selector.
Many semiconductor devices require voltage source selectors to select and output one of multiple input voltages. Among the more general class of voltage source selectors, a maximum voltage source selector is a circuit that selects a high power supply voltage among a plurality of power supply voltages to ensure stable operation of the constituent semiconductor device. For example, a maximum voltage source selector may be used to stably operate a power supply gate of a voltage boost circuit, such as a DC-DC converter.
A maximum voltage source selector is commonly used in semiconductor devices incorporated within portable electronic devices. Thus, the maximum voltage source selector enables the semiconductor device to stably operate by selecting the higher power supply voltage between an externally provided power supply voltage and, for example, a battery power supply voltage. For ease of reference and consistent with conventional use, the higher power supply voltage provided by the maximum voltage source selector, however derived, will be referred to as a “maximum voltage” without any intent to somehow suggest a conceptual or mathematical maximum for the signal or the plurality of signals from which the maximum voltage is selected.
Conventional maximum voltage source selectors select and output a maximum voltage through a different power voltage signal path depending on whether the incorporating semiconductor device is in an enable or disable state. Unfortunately, this use of alternate power voltage signal paths increased the overall area occupied by the maximum voltage source selector.